Pathological conditions resulting in cartilage degeneration constitute a major medical, social and economical problem. Of persons older than 65 years of age, 486 per 1,000 have arthritis. Traditionally, the clinical diagnosis of arthritis is based on the patient's history, physical examination, and radiographs. The prognosis, treatment, and clinical outcomes of patients with arthritis are assessed by serial examinations. In order to minimise permanent tissue damage caused by pathological conditions involving cartilage degeneration, it is important to be able diagnose such conditions at an early stage. However, many patients do not develop symptoms until late in the disease process. When a diagnosis is established, permanent tissue damage has often already been formed, and then it is no longer possible for the patient to recover completely.
Accordingly, there is a need for methods for diagnosing pathological cartilage degeneration based upon detection of biologic markers, which markers are released early in the disease progress. Much efforts have been made in finding suitable markers, and parts of this work are reviewed in Lohmander. Ballière's Clinical Rheumatology, vol. 11, p. 711-726; and in Scher et al., the American Journal of Orthopedics, April 1996, p. 263-272.
One possible marker that could be used in such a diagnosis method is COMP, Cartilage Oligomeric Matrix Protein. Elevated serum levels of COMP has previously been associated with joint destruction in rheumatoid arthritis (Månsson et al., J. Clin. Invest. (1995), vol. 95, pp. 1071-1077; Wollheim et al., British Journal of Rheumatology (1997), vol. 36, pp. 847-849; Petersson et al., British Journal of Rheumatology (1998), vol. 37, pp. 46-50). Significant amounts of small fragments of COMP have also been found in synovial fluid from patients with rheumatoid arthritis and other forms of inflammatory arthritis (Neidhardt et al., British Journal of Rheumatology 1997, 36: 1151-60).
It has also been suggested to use COMP or nucleic acid sequences encoding COMP for preparing a pharmaceutical composition for preventing and/or treating arthritic conditions in a mammal (WO 98/46253). Accordingly, COMP can be regarded as a key compound when diagnosing and/or treating different forms of arthritis.
Up till now, COMP has been analysed by an ELISA assay which is based upon polyclonal antibodies (Saxne et al., British Journal of Rheumatology (1992), vol. 31, pp. 583-591). A sample that is suspected to contain human COMP is added to polystyrene microtiter plates. The present COMP is then adsorbed to the plates. Rabbit antihuman COMP antibodies are added and are allowed to bind. Subsequently porcine anti-rabbit antibodies conjugated with alkaline phosphatase are added. A substrate for alkaline phosphatase, para-nitrophenyl phosphate, is finally added and the resulting absorbance at 405 nm can be regarded as a measurement of COMP.
Because of the fact that COMP is such an interesting pathological cartilage degeneration marker is very important to have access to an accurate and sensitive assay rendering it easy to make a distinction between healthy and pathological samples. Accordingly, there is a need for an improved assay method for determining the presence of COMP in a clinical sample.